Motors, such as permanent magnet motors, are used to provide mechanical power in a variety of applications, for example, to drive traction devices in transportation and construction applications. Motors are provided with a current to generate a mechanical torque output that may drive the traction devices. That is, currents within components (e.g. field windings) of the motor create an electromagnetic flux, and a rotating member of the motor is caused to rotate by interaction with the magnetic flux.
The current applied to a motor can be regarded as a vector with two components: a torque current component iq, and a flux current component id. By controlling these current components, the electromagnetic flux and output torque of the motor can be controlled. For example, an inverter can be controlled to apply one or more desired voltages to the motor, causing desired torque and flux current components to flow within the motor, thus inducing a desired mechanical torque output from the motor in accordance with known principles.
During normal operation of a motor, the motor will generate a counter-electromotive force (“back EMF”) that opposes the voltage applied to the motor. At certain speeds, the back EMF may become greater than the voltage applied to the motor, causing the motor to operate undesirably or to cease operation entirely. A method known as “field weakening” is employed to reduce the back EMF (i.e. maintain the back EMF at or below a desired level for operation of the induction motor) and allow the motor to operate at desired speeds.
Field weakening methods generally include reducing the flux of a motor when the motor speed increases beyond a predetermined speed threshold, and maintaining the flux of the motor at or above a normal flux level when the motor speed is below a predetermined threshold. Flux can be reduced by delivering corresponding voltage commands to an inverter. Field weakening methods can further include increasing the torque current component via corresponding voltage commands such that the mechanical torque output of the motor is maintained at or above a level necessary for efficient operation of the motor. A current regulator may be used to ensure that the correct voltage commands are being applied to the inverter.
During a field weakening method, the current regulator may be limited by the voltage on the DC bus. If an unachievable current command is provided to the current regulator, the current regulator may saturate, deteriorating its performance. Moreover, such a current command may trigger an over-current fault of the current regulator during motor deceleration. Therefore, a method is needed that will provide achievable current commands for controlling the current regulator.
One method of controlling a motor during a field weakening operation is described in U.S. Pat. No. 7,023,168 (the '168 patent) issued to Patel et al. on Apr. 4, 2006. The '168 patent describes a field weakening motor control system that includes a dominant feedforward stator flux value and a feedback flux correcting term. The two flux terms are added together and limited to a maximum flux at low speed that guarantees constant flux in a constant torque region.
Although the system of the '168 patent may increase the efficiency of a motor through field weakening, its reliance on feedback and feedforward flux values may be unnecessarily resource intensive. Because flux generally is not directly measured in a motor control system, the method proposed by the '168 patent requires calculating flux from other measured values. This may slow processing time by adding unnecessary calculations and/or components. Calculating field weakening commands based on indirectly calculated values may also reduce the accuracy of the field weakening commands.
The disclosed system is directed to overcoming one or more of the problems set forth above and/or other problems of the prior art.